Field
The disclosure relates to compact, high brightness, mode locked fiber laser based short pulse light sources and examples of applications thereof.
Description of the Related Art
Pulse sources based on mode locked fiber lasers are finding many applications in industry and medicine as well precision spectroscopy. The universality of this technology is exemplified by the many different applications that have been pursued comprising, heavy duty machining, precision ophthalmological surgery, medical imaging as well as precision metrology. Early passively mode locked fiber lasers were based on nonlinear amplifying loop mirrors (NALMs) as for example described in Fermann et al., ‘Additive-pulse-compression mode locking of a neodymium fiber laser’, Opt. Lett., Vol. 16, Issue 4, pp. 244-246 (1991). Another arrangement included a FIG. 8 laser (F8L) in order to initiate mode locking and to generate short optical pulses at high repetition rates, as disclosed in U.S. Pat. No. 5,365,531: “Apparatus and method for initializing an optical-fiber laser for mode-locking”. For commercial applications reliability and robustness of the mode locked lasers is very much a premium.
Previously, robust mode locked fiber laser light sources have been constructed using polarization maintaining components as well as saturable absorbers as for example disclosed in U.S. Pat. No. 7,088,756, “Polarization maintaining dispersion controlled fiber laser source of ultrashort pulses”. To maximize the output power from mode locked fiber lasers, previously mode locked operation in the similariton regime has been implemented as discussed in U.S. Pat. No. 7,782,910, “Single-polarization high power fiber lasers and amplifiers”, to Fermann et al. and U.S. Patent Application Pub. No. 2012/0205352, “Compact, coherent, high brightness light sources for the mid and far IR”.
To start the modelocking process, the use of ring laser cavities has been suggested as described in K. Tamura et al., ‘Unidirectional ring resonators for self-starting passively mode locked lasers’, Opt. Lett., vol. 18, pp. 220-222 (1993). Ring laser cavities are less susceptible to spurious intra-cavity reflections, which are generally believed to inhibit self-starting operation. According to recent theories, self-starting modelocking involves a noise activated crossing of an entropic barrier as described in A. Gordon et al., ‘Self-starting of passive modelocking’, Opt. Express, vol. 14, pp. 11142-11154 (2006). To cross the barrier to modelocking, laser cavities have been used that allow for Q-switching operation before transitioning to cw mode locked operation, as discussed in U.S. Pat. No. 6,956,887 ('887) and U.S. Pat. No. 7,453,913 ('913), “Resonant Fabry-Perot semiconductor saturable absorbers and two photon absorption power limiters”. To facilitate the evolution from Q-switching to modelocking and to prevent damage to optical components inside the laser, two photon peak power limiters have been implemented as also discussed in '887 and '913. The benefits of Q-switched operation (or sometimes referred to also as relaxation oscillations) in the transitioning to modelocking have recently been further confirmed by H. Li et al., ‘Starting dynamics of dissipative-soliton fiber laser’, Opt. Lett., vol. 15, pp. 2403-2405 (2010), but only for ring cavities.